1. Field of the Invention
The present invention relates generally to electrophotographic printing devices and, more particularly, to methods for operating the fuser in electrophotographic printing devices to reduce gloss discontinuity during duplex printing.
2. Description of the Related Art
In the electrophotographic (EP) imaging process used in printers, copiers and the like, a photosensitive member, such as a photoconductive drum or belt, is uniformly charged over an outer surface. An electrostatic latent image is formed by selectively exposing the uniformly charged surface of the photosensitive member. Toner particles are applied to the electrostatic latent image, and thereafter the toner image is transferred to the media intended to receive the final permanent image. The toner image is fixed to the media by the application of heat and pressure in a fuser.
A fuser is known to include a heated roll and a backup roll forming a fuser nip through which the media passes. During the fusing process, it is necessary that sufficient heat be applied to the toner particles so that the toner is permanently affixed to the media. Adequate fusing temperatures are quite high, and even relatively minor variations in the temperature around the circumference of the heated roll can alter the gloss appearance of the final image. Therefore, it is necessary to maintain the heated roll at a substantially consistent temperature over the entire surface thereof. If a portion of the media-contacting surface of the heated roll is cooler than other portions, the image on the media can have visually noticeable dull spots that are less glossy than other areas that received higher temperature during fusing. When the hot roll and backup roll are turned continuously, the surfaces thereof retain substantially consistent temperatures around the circumferences of each. Maintaining substantially consistent surface temperatures becomes more difficult as process speeds increase and there is less time for temperature equilibration between fusing operations on successive pieces of media.
To reduce printer size and cost while retaining high output performance, it is known to use printer architecture in which duplex routing includes passing the media nearly into the output bin before rapidly withdrawing the media back into the duplex path for imaging the second side. Two motors can be used, one to operate the fuser in the process direction, and a second to drive the output rolls in reverse to withdraw the sheet from the output area. To further reduce machine costs, a single reversible fuser motor can be used. For duplexing, the motor is reversed from the normal process direction when the media is withdrawn from the output area and directed to the duplex path. Since duplex routing essentially is “dead time” during which no fusing operation occurs, it is desirable to reduce the time required to reverse the sheet to a period as short as possible. Therefore, during duplex routing, it is desirable to operate the motor at higher speed than normal process speed. This can be accomplished by using a motor of sufficient size to reverse quickly and drive all fuser components at a faster speed in reverse than in the normal process direction. However, this adds significant cost for a larger motor that is required for a brief time only, and only when duplex printing is used.
It is proposed to disengage the fuser rolls when the motor is reversed, thereby decreasing the load inertia on the motor, and allowing the motor to reverse more quickly and thereby increase duplex throughput. A suitable structure for disengaging the fuser rolls is a swing arm assembly that disengages the hot roll gear from the fuser drive train when the motor is reversed. However, when the heated roll and the pressure roll are stopped in contact with each other, significant heat transfer occurs through the nip, from the hot roll to the backup roll. As a result, a cold spot occurs on the hot roll, which can cause horizontal bands of gloss discontinuity on the printed media. Since the change in gloss is relatively abrupt, it can be noticeable on solid images particularly.
It is known to use so called multi-mode duplexers that can alter the manner in which duplex printjobs are performed. In a three-image duplexer, three pages are in the paper path at one time. In a two-image duplexer, two pages are present in the paper path at one time. In a one-image duplexer, only a single page is in the paper path at any time. A multi-mode duplexer can switch between various multi-image processes or to a one-image process, in response to the complexity of the images and the amount of memory available.
What is needed in the art is an operating process to improve temperature consistency around the circumference of the fuser rolls.